The present invention relates to the field of lithography and more specifically relates to the use of chemically amplified photoresists in lithography.
Semiconductors are widely used in integrated circuits for electronics applications, including information systems. Such integrated circuits typically employ transistors and multiple levels of device interconnects fabricated in silicon. Various device layers may be sequentially formed on a semiconductor wafer using a combination of microlithography and etch processes.
Microlithography is a commonly practiced process of creating a patterned mask on the surface of a semiconductor wafer so that subsequent patterned processes may be performed. Typically these subsequent patterned processes involve the addition or subtraction of a material by deposition, implant doping, or plasma etching. Frequently, the pattern is transferred from an exposure mask to the wafer using a photoresist layer and optical lithography exposure tools.
Many modem semiconductor fabrication processes involve the deposition of a photosensitive resist material upon a substrate such as a wafer that may have various material layers formed upon it. The resist material is then exposed to radiation of a particular frequency. The radiation interacts with the resist material and produces a pattern within the resist, termed a xe2x80x9clatent image.xe2x80x9d
There is a desire in the industry for higher circuit density in microelectronic devices that are made using lithographic techniques. One method of increasing the number of components per chip is to decrease the minimum feature size on the chip, which requires higher lithographic resolution. The use of shorter wavelength radiation (e.g., xe2x80x9cdeepxe2x80x9d or xe2x80x9cextremexe2x80x9d ultraviolet (UV), in the range of from about 190 to about 315 nm) offers the potential for higher resolution. However, with deep UV radiation, higher exposure doses may be required to achieve the desired photochemical response.
As the exposure wavelength of modern microlithographic tools continues to decrease, chemically amplified photoresists are becoming increasingly important. Several acid catalyzed chemically amplified resist compositions have been developed. Chemically amplified resist compositions generally comprise a photosensitive acid (xe2x80x9cphotoacidxe2x80x9d) generator (PAG) and an acid sensitive polymer. Upon exposure to radiation (e.g., x-ray radiation, ultraviolet radiation), the photoacid generator, by producing a proton, creates a photogenerated catalyst (usually a strong acid) during the exposure to radiation. During a post-exposure bake (PEB), the acid may act as a catalyst for further reactions. For example, the acid generated may facilitate the cross-linking in the photoresist. The generation of acid from the PAG does not necessarily require heat. However, many known chemically amplified resists require a post-exposure bake (PEB) of one to two minutes in length to complete the reaction between the acid moiety and the acid labile component. During this time, acid diffusion in the film can cause an undesirable effect if acid moieties migrate into unexposed regions.
Issues such as acid diffusion during bake steps can affect critical dimension and linewidth variation in a semiconductor. Accordingly, knowledge of the location, amount, and extent of diffusion of the photogenerated acid in the photoresist is crucial for understanding resist behavior. Since it can be extremely difficult to make optical contact with the acids directly, their location is generally inferred from scanning electron microscopy (SEM) images of developed patterns. These exposed images, however, are convolved with subsequent processes such as resist chemistry, baking, and chemical development. For this reason, it is desirable to have a method of detecting latent images in exposed photoresists which allows direct determination of acid location (i.e., without requiring additional baking or developing processes).
Previous studies of latent images have been undertaken with a variety of methods, including atomic force microscopy (see Ocola et al, Appl. Phys. Lett. 68, 717 (1996)), and photon tunneling microscopy (see Marchman and Novembre, Appl. Phys. Lett. 66, 3269 (1995); Liddle et al., J. Vac. Sci. Technol. B 15, 2162 (1997)). These techniques rely on contrast mechanisms resulting from topographic and/or refractive index variations in exposed resist. It would be desirable, however, to have an imaging technique that is sensitive directly to the presence of the photogenerated acid molecules.
There is also a need to be able to evaluate whether or not a photoacid generator present in a chemically amplified photoresist is efficient (i.e., produces sufficient acid to catalyze the desired reaction). Similarly, there is a desire to be able to compare the efficiency of one photoacid generator to another. It is also desirable to be able to quantify the number of acid molecules generated by the photoacid generator upon exposure to radiation. Presently, these kinds of determination may be made by spectrophotometric titration (i.e., comparative on-wafer absorbance measurements). However, this kind of measurement is generally very time consuming, and cannot be made with certain incident radiation (e.g., at 193 nm). A method that may be used with a broad range of radiation wavelengths, and which significantly reduces the amount of time required to make these determinations, is desirable.
It is an object of the invention to provide methods for imaging acids in a chemically amplified photoresist composition prior to post-exposure bake.
It is also an object of the invention to provide methods for imaging an acid in a chemically amplified photoresist by directly determining the location of the acid within the resist composition.
It is additionally an object of the present invention to provide methods for comparing the efficiency of photoacid generators.
It is yet another object of the invention to provide chemically amplified photoresist compositions that may be used to image the location of the acid generated in the photoresist composition during exposure to radiation.
It is yet another aspect of the invention to provide methods for measuring acid diffusion within a photoresist composition.
Accordingly, the present invention provides a novel method of imaging acid in a chemically amplified photoresist, by exposing to radiation a chemically amplified photoresist that generates acid when exposed to the radiation. The chemical amplified photoresist comprises at least one pH-dependent fluorophore that fluoresces in the presence of acid and when exposed to the radiation. The chemically amplified photoresist comprising a pH-dependent fluorophore may be made by adding an amount of pH-dependent fluorophore to a casting solution of a known chemically amplified photoresist composition. An image of the acid in the photoresist is then generated, preferably by fluorescent imaging. The chemically amplified photoresist may be applied to as substrate such as a silicon photoresist prior to exposure to radiation. The radiation may be ultraviolet (UV) radiation (including deep UV radiation), x-ray, or any other known means of radiation. The pH-dependent fluorophore is preferably a rhodol derivative, although any pH-dependent fluorophore is useful in the practice of the present invention. The imaging of the acid in the chemically amplified photoresist provides a map of the location of acid generated in the photoresist.
The present invention also provides a method of making a chemically amplified photoresist, comprising admixing a polymeric resin, a photoacid generator, and a pH-dependent fluorophore. The polymeric resin may be, and preferably is, a novolak or novolak-based resin. The pH-dependent fluorophore is derivative of rhodol, and is more preferably one of the pH-dependent fluorophores Cl-NERF or DM-NERF.
The present invention further provides novel chemically amplified photoresist compositions that may be used for the detection and imaging of acids in the chemically amplified photoresist. These chemically amplified photoresists comprise a polymeric resin, a photoacid generator, and a pH-dependent fluorophore.
The invention also provides a method of measuring the amount of acid generated by a photoacid generator in a chemically amplified photoresist composition when the chemically amplified photoresist is exposed to radiation. This method involves exposing the chemically amplified photoresist composition to radiation, the photoresist composition comprising a photoacid generator and a pH-dependent fluorophore that fluoresces in the presence of an acid and when exposed to the radiation. The amount of the fluorescence generated by the chemically amplified photoresist is then detected with the amount of fluorescence correlating with the amount of acid generated by the photoacid generator.
A fluorescence-based technique for mapping pH gradients in chemically amplified photoresists is accordingly disclosed herein. The methods and compositions described herein are particularly advantageous in that they provide a way to directly measure the location and amount of acid in a chemically amplified photoresist. This feature allows the practitioner a spatial way to, for example, control the diffusion of acid into the chemically amplified photoresist. For example, if the method of imaging the acid in the photoresist indicates that diffusion of acid into the resist composition is to great, the practitioner may change the components in resist, or the amounts thereof, to limits the acid diffusion. Such control allows the practitioner a way to improve resolution in electronic microdevices such as, for example, integrated circuits and semiconductors. Accordingly, the present invention is useful in the study of semiconductors generally, and in the study and optimization of semiconductor fabrication specifically.
This invention is particularly advantageous in that it allows for the study of latent images formed in the resist, after exposure to radiation but without baking (i.e., PEB) or developing the resist, for almost any lithographic technique used in the semiconductor industry today. In addition, this technique has the potential for the rapid determination of photogenerated acid yield among a variety of photoacid generators. Due to the relatively low level of pH-dependent fluorophore required and high signal to noise ratio, the methods may also be used for photoacid yield determination in resists without altering the absorbance characteristics of the film. This is especially important in 193 nm lithography, or exposures using extreme ultraviolet lithography where the exposure depth is only about 200 nm.
These and other aspects and object of the invention, and the equivalents thereof, are described in further detail in the drawings and descriptions that follow.